In a typical satellite system, either an areawide (e.g. nationwide) antenna beam or narrow zone beams are employed for communications. In Ku band, the satellite communication band most suitable for two-way service to very small terminals, the attenuation of the signals by rain is an important design consideration. The rain attenuation is overcome on the downlink by using higher satellite transmission power per channel than would be necessary for clear weather service, typically four times as much. This approach to accommodation of rain attenuation therefore results in more expensive satellites having fewer available channels.
Typically, the geographic region covered by the satellite is divided into zones with one downlink antenna beam being dedicated to each zone. When zone downlink beams are employed, usually each transmitter is associated with each zone and cannot be simultaneously used with any other zone. Although the use of zone beams is advantageous in that zone beams have high antenna gain, these systems do not sufficiently accommodate disadvantaged downlink users. If a downlink user is located in an area experiencing heavy rainfall, in order to compensate for the rain attenuation, the uplink signal has to be made more powerful. Since each of the zone beams has a limited amount of power associated with it (i.e. the amount of power from its transmitter), the extra power needed to compensate for rain comes from decreasing the amount of power provided to the rest of the downlink users in that particular zone. Since the unattenuated users in the zone receive less power, the number of available channels in that particular zone decreases because there is not enough power to supply all the users.
Conversely, systems employing one nationwide antenna beam have a nationwide pool of power available to them because the nation is served by all of the transmitter. To compensate for rain attenuation, the uplink power can be increased without overtaxing the other users located throughout the nation. This is because the area over which heavy rain occurs at any instance, is small compared to the area of the entire nation. Hence the average penalty to the impaired users decreases since their loss is spread out over a larger number of unimpaired users. Despite having a pool of power available, a nationwide beam is not ideal for two-way communications because a wide antenna beam means low antenna gain, and for communication to very small earth terminals, a high gain antenna beam is highly desirable.